1. Field of the Invention
This invention relates to a catalytic reforming process wherein a suitable charge stock, such as petroleum naphtha, is converted to a gasoline of high octane number under conditions which involve the presence of introduced sulfur into the reaction zone to counteract declining production of C.sub.5 + product and where the catalyst contained in said zone consists essentially of separately supported platinum and separately supported iridium particles.
2. Description of the Prior Art
Catalysts intended for use in reforming operations wherein hydrocarbon fractions such as naphthas or gasolines or mixtures thereof are treated to improve the anti-knock characteristics thereof are well known in the petroleum industry.
It has heretofore been proposed to employ platinum metal-containing catalysts for promoting reforming. Such catalysts are necessarily characterized by a certain amount of acidity. One type of reforming catalyst which has been used commercially consists of an alumina base material having platinum metal impregnated thereon, with the acidity characteristic being contributed by a small amount of halogen incorporated in the catalyst.
In more recent years, multimetallic reforming catalysts, for example, bimetallic catalysts, have come into use. These catalysts generally contain platinum, together with one or more additional metals such as rhenium, germanium, iridium, palladium, osmium, ruthenium, rhodium, copper, silver, tin or gold deposited on a refractory support which also contains a specified amount of halogen. Representative of multimetallic reforming catalysts are those containing platinum and iridium, such as described in U.S. Pat. No. 2,848,377 and more recently in U.S. Pat. No. 3,953,368. The latter patent reports certain advantages when platinum and iridium are present on a refractory support as highly dispersed polymetallic clusters in which metallic atoms are separated by distances of about 2.5 to 4.0 Angstroms.
It has also heretofore been known to conduct catalytic reforming utilizing a catalyst consisting essentially of a particularly defined mixture of particles on a porous carrier impregnated with a small amount of platinum and particles of an acidic cracking component. Representative disclosures of such prior art are to be found in U.S. Pat. Nos. 2,854,400; 2,854,403; and 2,854,404. Also, it has been suggested, for example, in German O.S. No. 2,627,822 to conduct reforming in the presence of a catalyst constituting a mixture of platinum on one solid carrier and rhenium on a second solid carrier.
All of the above catalysts, while possessing certain advantages, suffer from inherent deficiencies in either activity, stability or sensitivity to poisons, e.g., sulfur, under severe reforming conditions.
During startup of a reforming operation, excessive amount of light gases, e.g., methane and ethane, are normally produced unless proper pretreatment procedures are utilized. The light hydrocarbon gases, produced as a result of high hydrocracking activity or metal-cracking activity of the catalyst, are particularly to be avoided during reforming since they decrease the yield of gasoline boiling products. Hydrocracking activity can be diminished if the catalyst is sulfided prior to contact with the charge stock. The presulfiding can be accomplished, for example, by passing a sulfur-containing gas, e.g., H.sub.2 S, through the catalyst bed. Other presulfiding treatments are known in the prior art. It has also been known that on startup and during operation, a small amount of sulfur added to the reforming zone effectively reduces the initial hydrocracking activity of the catalyst.
It is disclosed in U.S. Pat. No. 3,554,902 that intermittent sulfiding of a platinum-iridium catalyst during reforming improves the yield of gasoline product. Thus, as the yield of C.sub.5 + product begins to decline during the reforming process, it has been reported that injection of a slug of sulfur into the reaction zone results in an increase in the yield. It is indicated that generally no more sulfur should be injected at a time than triple the theoretical amount to sulfide the platinum and iridium present on the catalyst.
It has also heretofore been reported, for example, in U.S. Pat. No. 3,507,781 that reforming catalysts comprising catalytically active amounts of platinum and iridium, deposited by coimpregnation of a solid porous support, such as alumina, are extremely sensitive to the presence of sulfur in the charge stock.